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What is the Fab layer used for in PCB design? What should it include?

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    \$\begingroup\$ "The fabrication layers are used to display the simplified mechanical outline of components on the PCB" - says a very quick Google search. \$\endgroup\$
    – Finbarr
    Jul 25, 2022 at 10:18

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Fab is short for "fabrication" (not "fabulous" as you might have guessed).

There's no real requirement for it to contain anything or for it to be sent to the PCB maker. There are imposed standards or at least suggestions in various domains.

One example of the use is KiCad's library rules:

F5.2 Fabrication layer requirements The fabrication layers are used to display the simplified mechanical outline of components on the PCB.

KiCad refers to the fabrication layers as:

F.Fab - Front fabrication layer

B.Fab - Back fabrication layer

The following elements must be provided on the fabrication layer(s)

Simplified component outline must be provided on F.Fab layer

Outline uses line width between {0.10mm and 0.15mm} (recommended 0.10mm)

Outline should be simplified and not display complex features

For outlines based on the component body shape, the nominal size is used

Footprint polarisation / location of pin-1 is drawn

For IC packages, bevel is drawn at corner next to pin-1

Bevel should be 1mm or 25% of package size (whichever is smaller)

For connectors, a small arrow indicator drawn next to pin-1 should be used

Component value (footprint name) must be displayed on the F.Fab layer

Recommended text size = 1.0mm

Allowable text size = {0.5mm to 1.0mm}

Text thickness should be approximately 15% of text size ( with allowances for variation for aesthetic reasons)

Placed below (positive y direction) the part outline.

A second copy of the reference designator (RefDes) must be provided on the F.Fab layer. To add a second RefDes item, add a text object with the value ${REFERENCE}

RefDes must be centered on component body (inside component outline)

Orientation of RefDes should match major component axis

Size of text should be scaled to match component size

It is recommended to scale it such that 4 characters fit without overlapping other features of the same layer.

If it is not possible to fit at least 3 characters with the text size restrictions, then the reference should be moved outside (but scaled to smallest allowable text size)

Recommended text size = 1.00mm

Allowable text size = {0.5mm to 1.0mm}

Text thickness should be approximately 15% of text size (with allowances for variation for aesthetic reasons)

Others think it should have a layer stack legend and dimensioned drawing of the board outline, drill drawing, drill table and such like.

Pretty radically different views.

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The Fab layer is for your fabrication drawing. Put everything on it that you think might be useful during fabrication and assembly.

I like to put the PCB outline, component outlines, component designators, mounting holes, layer stackup information and the size of the board on the fab layer.

This gives me a nice drawing from which I can check and assemble the PCB.

There are different schools of thought on what a fab drawing must contain and it always depends on who will have to use it...

Here is a nice writeup.

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  • \$\begingroup\$ Isn't all of that inforamtion in the other layers? Why put them all in one layer? \$\endgroup\$
    – manarinian
    Jul 25, 2022 at 15:07
  • \$\begingroup\$ @manarinian no, it isn't. There is no information about layer stackup or size of the outline anywhere else. You might have some information on the silkscreen but that's not complete in most cases for many different reasons. And it's often just more convenient to have it all in one place. \$\endgroup\$
    – kruemi
    Jul 26, 2022 at 4:56

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